Add method of requiring that the user specify the API version they'd like to use

If the controller overrides default_version and has it return nil, then the caller will get a
404 if they do not specify the API version in the request (via the header or in the URL if using
the middleware).
Add .ruby-version to .gitignore
Merge pull request #2 from centro/require_version

Add way of requiring user to specify API version being invoked